In a power supply device, there is an insulated AC-DC converter, which includes a voltage converting transformer, converts a voltage of alternating current power, rectifies an alternating current induced on a secondary side, and converts the alternating current into a direct current voltage at a desired potential. As the insulated AC-DC converter, for example, there is known a switching power supply device configured to control a voltage to be induced in a secondary side winding by performing switching control for a current flowing through a primary side winding of the voltage converting transformer.
Incidentally, in the switching power supply device, it is frequent that pulse width modulation (PWM) control is adopted in order to enhance electric power efficiency. Also in the insulated AC-DC converter, there is proposed an invention configured in such a way that a detection signal of a secondary side output is fed back to a primary side control circuit by a photo coupler, and that the primary side control circuit performs ON/OFF drive for the switching element by a PWM pulse to control a current flowing through a primary side coil (refer to Patent Document 1, for example).